DE 198 41 159 A1 describes generic drive units for motor vehicles. One of the drive units has an electric motor and a transmission assembly which are operatively connected to one another by means of at least one shaft. The transmission assembly comprises a differential and is in operative connection with the electric motor by way of a planetary drive of the transmission assembly. The electric motor is arranged coaxially in relation to the differential. The differential is driven by the electric motor. For this reason, a transmission element is connected to the rotor shaft of the electric motor for rotation therewith.
The rotor shaft is rotatably mounted on a stator carrier by means of two rolling bearings in the form of ball bearings. The stator carrier is formed as one part with part of a housing of the drive unit. The rotor shaft is provided on the differential side with a gearing, which is the sun wheel of a planetary drive. The planet carrier for the planet wheels, which are in engagement with the sun of the planet wheel, is connected to the cage of the differential. The axle drive bevel gears of the differential are respectively connected to an output shaft for rotation therewith. One of the output shafts is arranged concentrically in relation to the input shaft, i.e. in relation to the rotor shaft and in relation to the rotor of the electric motor. For this purpose, the output shaft is fitted in the rotor shaft, which is formed as a hollow shaft, and is slidingly mounted in it. The transmission assembly is separated from the electric motor by a housing cover. The housing cover has the rotary lead-through for the rotor shaft, and consequently also for the output shaft fitted therein.
The interior of the electric motor is sealed off at the rotary lead-through with respect to the transmission assembly by means of a rotary shaft seal. Rotary shaft sealing rings have at least one sealing lip, which is in physical contact with a sealing surface and accordingly seals in a sliding manner during rotation. Both rolling bearings for mounting the rotor shaft are located inside the electric motor, which is not exposed to lubricating oil. This “dry” area is separated from a “wet” transmission-side area of the drive unit by means of a rotary shaft seal. The “wet” area has lubricating oil, which is distributed as spray oil or oil mist. Since the rolling bearings are arranged in the dry area, they are lubricated with grease and sealed off. The rolling bearings are ball hearings.
The lifetime of a rolling bearing is dependent on the service life of the lubricating grease with which this rolling bearing is lubricated. The service life of the grease is dependent on various factors, such as the type of lubricating grease, the rotational speed and the temperature. High temperatures in the electric motor may have disadvantageous effects on the service life of the grease, and consequently on the lifetime of the bearing.
DE 198 41 159 A1 shows with FIG. 2 a further drive unit of the generic type in which, as a departure from the arrangement described above, the rotor shaft is mounted by means of bail bearings on the housing of the drive unit, and consequently not inside the electric motor. One of the ball hearings is located on the transmission side in a housing cover which separates the electric motor from the transmission assembly. The other of the ball bearings is located on the output side in the electric motor on a portion of the housing. The housing cover has the rotary lead-through for the rotor shaft. The “dry” interior of the electric motor is sealed off at the rotary lead-through with respect to the “wet” area of the transmission assembly by means of a rotary shaft seal. The transmission-side ball bearing is located in the “wet” area and is lubricated with lubricating oil. The lubricating oil is directed to the ball bearing by way of an annular gap between the rotor shaft and the housing cover. In the rotary lead-through, the rotary shaft seal follows behind the ball bearing on the electric motor side. The lifetime of this ball bearing is strongly dependent on the amount of lubricating oil that constantly reaches the interior of the ball bearing.
The arrangement according to DE 198 41 159 A1 in FIG. 2 also shows a rotor shaft, which is a hollow shaft and in which an output shaft is fitted. An annular gap between the output shaft and the rotor shaft is sealed off on the transmission side toward the annular gap by means of a rotary shaft sealing ring, which prevents oil from escaping from the transmission by way of the annular gap.